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UNIT 4 Electromagnetic Induction and a.c.
Recommended Prior Knowledge It is essential that A2 Unit 3 is studied before this Unit.
Context This unit is a continuation of the work on magnetic fields to study aspects of electromagnetic induction. An introduction to alternating current is also
included.
Outline The concepts of flux and flux linkage are introduced so that electromagnetic induction may be studied. Aspects of alternating current are also
included.
Online Resources. In this unit repeated use is made of Java Applets. These are usually well animated programmes. It has been found preferable to use one
main source of computer programmes as both teachers and students can become familiar with the procedure for using the resource and are therefore more
likely to connect to the web. The set of animations using the web address http://surendranath.tripod.com do not contain any applets on electromagnetic
induction or a.c. but once this address has been found then the Applet menu labelled ‘OTHER’ enables Applets on this topic from other sources to be found.
There is no need to subscribe to Tripod, just ignore their adverts and click on to either the Applets Menu or the ‘Click here’ instruction about any problem with
the menu. In the Online Resources column reference will be made directly to different authors but payment is required for applets called ‘Physlets’.
23(a)
23(b)
23(c)
23(d)
Learning Outcomes
Electromagnetic Induction
Define magnetic flux and the weber.
Recall and solve problems using F = BA.
Define magnetic flux linkage.
Infer from appropriate experiments on
electromagnetic induction:
(i) that a changing magnetic flux can
induce an e.m.f. in a circuit,
(ii) that the direction of the induced e.m.f.
opposes the change producing it,
Suggested Teaching Activities
Online Resources
This topic is dealt with by
the following books
It is helpful to use the alternative unit for
-2
magnetic flux density (A2 Unit 3) – Wb m
and to model field strength as the number
of lines normal to unit area. It may be
advantageous to consider 23(c) as part of
23(d)(i).
Demonstrations should include:
moving a wire between the poles of a
horseshoe magnet
moving a wire in the region of a magnet
moving a magnet in/out of a coil
moving a coil in the region of a magnet
switching a current on/off in a coil, inducing
an e.m.f. in a second coil
using a ferrous core in the coils
The demonstrations above allow the
direction of the induced current to be
determined so that it can be deduced that a
magnetic field is induced that opposes the
change. This is formalised as Lenz’s law .
www.xtremepapers.net
Other resources
Understanding Physics for
Advanced Level: Breithaupt
ISBN: 0748743146
Physics: Hutchings
ISBN 0174387318
http://webphysics.davids
on.edu/physlet
then go to
Part 5:
Electromagnetism and
on to Chapter 29:
Faraday’s Law.
Physics 2: Sang (Editor)
ISBN 0521797152
Advanced Physics: Duncan
ISBN 0719576695
Practice in Physics: Akrill
ISBN 0340758139
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(iii) the factors affecting the magnitude of
the induced e.m.f.
23(e)
23(f)
24(a)
24(d)
24(b)
24(c)
24(e)
24(f)
Recall and solve problems using Faraday’s
law of electromagnetic induction and Lenz’s
law.
Explain simple applications of
electromagnetic induction.
Alternating currents
Show an understanding and use the terms
period, frequency, peak value and rootmean-square value as applied to an
alternating current or voltage.
Distinguish between r.m.s. and peak values
and recall and solve problems using the
relationship I = I0 / Ö2 for the sinusoidal
case.
Deduce that the mean power in a resistive
load is half the maximum power for a
sinusoidal alternating current.
Represent an alternating current or an
alternating voltage by an equation of the
form x = x0sinwt.
Show an understanding of the principle of
operation of a simple iron-cored transformer
and solve problems using
Ns/Np = Vs/Vp = Ip/Is for an ideal
transformer.
Show an appreciation of the scientific and
economic advantages of alternating current
and of high voltages for the transmission of
electrical energy.
The above demonstrations can be used to
deduce that rate of cutting of flux is the
important factor.
Flux linkage can be introduced.
Faraday’s law is the formal statement.
e.g. aircraft wing
change of flux in a coil
closing a metal window
e.g. mutual inductance ® transformer
shaver socket
moving coil a.c. generator (no theory)
Students should appreciate that the r.m.s.
value (rating) of an alternating current is
that value of the direct current that
produces thermal energy in a resistor at the
same rate as the alternating current.
The insulation required on a 240 V mains
cable should be calculated
The fact that this may be considered as
another form of oscillations should be
discussed.
This should be considered as an example
of e.m. induction. The input and output
frequencies should be discussed.
It should be understood that the relations
are a consequence of no loss in flux and
100% efficiency.
This may be approached as a discussion
and should include
power losses in transmission at high/low
voltages
ease of changing voltage in home etc
www.xtremepapers.net
9702/4 May 02, 6(a)
http://www.walterfendt.de/ph14e
then go to
Electrodynamics and on
to Generator
http://www.walterfendt.de/ph14e
then go to
Electrodynamics and on
to Simple AC circuits
http://webphysics.davids
on.edu/physlet
then go to
Part 5:
Electromagnetism and
on to Chapter 31: AC
Circuits.
http://micro.magnet.fsu.
edu/electromag/java/ind
ex.html
then to ‘How a
transformer works’
9702/4 Nov 02, 7(a)
9702/4 May 02, 6(b)
9702/4 Nov 02, 7(b)
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24(g)
24(h)
24(i)
24(j)
Distinguish graphically between half-wave
and full-wave rectification.
Explain the use of a single diode for the
half-wave rectification of an alternating
current.
Explain the use of four diodes (bridge
rectifier) for the full-wave rectification of an
alternating current.
Analyse the effect of a single capacitor in
smoothing, including the effect of the value
of capacitance in relation to load resistance.
The waveforms should be shown on a c.r.o.
Only the use of ideal diodes is expected.
Candidates are expected to be able to draw
a bridge-rectifier circuit.
9702/4 May 02, 6(b)
It is not expected that students will
understand the concept of time constant
CR. However, it is useful for them to know
that, for smooth d.c., CR should be much
larger than the period of the alternating
waveform.
9702/4 May 02, 6(b)
www.xtremepapers.net